In magnetic resonance (MR) imaging systems, a main magnet is used to generate a strong static magnetic field, which is homogenous in a volume of interest. The volume of interest corresponds to an examination space of the MR imaging system and is typically a spherical or ellipsoid space with a diameter of about 50 centimeters.
Typically, two types of radio frequency (RF) coils are used for exciting nuclear spins within a subject of interest, usually a patient, and detecting signals from them. Birdcage coils and transverse electromagnetic (TEM) coils are widely used for MR imaging in the very-high RF band (VHF) and have been introduced in commercial 3 T MR imaging scanners. The RF coils can be used as transmit and receive coils, as transmit only coils, or as receive only coils.
The RF coils may comprise different coil elements, which have at least one feeding port each for providing single channel information. When used as receive coil, each connection port provides an analog RF signal, also referred to as analog MR information signal, which has to be processed within the MR imaging system. The analog MR information signal is processed and reconstructed to finally obtain an MR image, as described in detail below.
A partitioning of components used in state of the Art MR imaging systems for processing the MR information signal is shown in FIG. 1. The MR imaging system 1 comprises four functional blocks, which are an acquisition block 2, a digitization block 3, a processing block 4, and a reconstruction block 5. As can be seen in FIG. 1, these blocks 2, 3, 4, 5 are provided in different locations. The acquisition block 2 is provided close to the subject of interest, where the magnetic resonance signals are acquired. The digitization block 3 is located within an exam room and converts the analog MR information signal into a digital MR information signal. The exam room is usually build to surround the acquisition block 2, i.e. the exam room also contains required magnets and coils of the MR imaging system 1. Nevertheless, the digitization block 3 is located apart from the acquisition block 2 within the exam room. The acquisition block 2 and the digitization block 3 are typically connected by means of coaxial cables 6 to transmit the analog MR information signal. The processing block 4 and the reconstruction block 5 are located in a tech room, which is separate from the exam room. The digitization block 3 is connected by means of digital cables 7 to the processing block 4, which is also connected by means of digital cables 7 to the reconstruction block 5. The digital cables 7 are provided e.g. as optical cables.
FIG. 2 shows in detail the acquisition block 2 and the digitization block 3 with an intermediate multiplexing block 8. The acquisition block 2 in this state of the Art MR imaging system 1 is implemented as integral RF antenna device and comprises two RF coils 12, each having a connection port 14, at which the analog MR information signal is provided. It is to be noted that the analog MR information signal in this state of the Art MR imaging system 1 comprises two individual signals. The acquisition block 2 further comprises two low noise amplifiers 16 for amplifying the analog MR information signal from the two RF coils 12. The amplified MR information signal is propagated in an analog way via coaxial cables 6 to the intermediate multiplexing block 8. The multiplexing block 8 is located in the exam room close to the acquisition block 2 to facilitate and reduce cabling of the coaxial cables 6. The multiplexing block 8 performs analog multiplexing of the two individual signals of the amplified analog MR information signal, and provides an analog multiplex MR information signal. The analog multiplex MR information signal is transferred from the multiplexing block 8 via coaxial cable 6 to the digitization block 3. The digitization block 3 is located away from the acquisition block 2 within the exam room, as described above. The digitization block 3 comprises a receive amplifier 21, a discrete band-pass filter 22 and an analog to digital converter (ADC) 24, which is provided as a converter IC. The analog multiplex MR information signal is filtered with the band-pass filter 22 and converted directly at the RF frequency with the ADC 24, which is a wideband ADC. In state of the art implementations, a commercially available ADC is used. However, due to a relatively high carrier frequency of the RF signal, which is typically about 100 MHz, and a required ADC resolution, the ADC is used in sub-sampling mode, where the sampling frequency of the ADC 24 is lower than two times the Nyquist frequency of the MR information signal. In order to implement a sufficient dynamic range, two ADCs 24 are used in a parallel configuration for coarse and fine conversion. Such a MR imaging system 1 and RF antenna device are e.g. known from WO 2006/048816 A1.
This implementation leaves possibilities for further improvements. First, the propagation of the high frequency analog MR information signal to the electronic processing unit requires RF cables with an excellent shielding due to the strong magnetic and/or RF fields used in MR imaging systems, which complicates the cabling. Typically, cable trapping of common mode currents has to be used, which further increases the cost of the implementation and reduces the reliability of the whole system. Furthermore, a sharp analog band-pass filter is required, which requires a high effort in providing the filter. Also, the electronics, in particular the two broadband ADCs, has significant power consumption. In addition, there are high requirements for analog signal matching and equalization in order to assure a linear dynamic range of the signal processing. Still further, a large volume or area is occupied by the electronics of the different blocks due to their big form factor. The implementations of the blocks can only be provided with a relatively low level of integration.
With new and emerging applications of MR imaging (MRI), frequently an increased number of channels is required, which additionally increases the cost and complexity of the MR imaging system, in particular the complexity of the analog cabling and the electronics of the multiplexing block and the digitization block is increased.